*2.3.1 The aromatic precursors*

Riboflavin present in wine is likely the most important precursor of the light struck taste default. In food and beverages, riboflavin is naturally present as flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and riboflavin (RF). In wine only RF form has been detected [26]. In bottled wines, RF participates in light-induced reactions that affect changes in volatile compounds, color, and flavor [4, 26]. When the RF concentration in wine is greater than 100 μg/L, the wine is considered to have a high risk of presenting the LST [27]. RF is a highly photosensitive compound which can be degraded in the presence of fluorescent or phosphorescent light, with wavelengths ranging from 370 to 450 nm. Its photochemical degradation can follow several paths of which intermolecular photo-reduction is the most relevant. The first step in the degradation mechanism is the uptake of a pair of electrons from an external donor (in this case methionine) by riboflavin. By this way a reduced flavin and methional is obtained. Methional is extremely unstable and breaks down to form methanethiol and acrolein. And the reaction of two methanthiol molecules can produce dimethyl disulfide [28]. The interconversion of diethyl disulfide and ethanethiol in presence of sulfites was also reported [29]. Although the rate of reaction is slow at wine pH, model predictions indicate that the reduction of diethyl disulfide to ethanethiol over time can be of sensory importance in wine [29].

RF plays a fundamental role in the oxidation of sulfur amino acids such as methionine and cysteine. Strecker's degradation of amino acids such as methionine and cysteine to aldehydes by α-dicarbonyl compounds formed during fermentation or oxidation contributes to the evolution of the aroma in bottled wine [30]. Glyoxal, and α-dicarbonyl compound generated during alcoholic and malolactic fermentation, reacts with methionine to form methanethiol and dimethyl disulfide, and with cysteine to form hydrogen sulfide, methanethiol, and other compounds [31, 32].

Wine is made up of a large number of different amino acids and, among them, methionine and cysteine are also important precursors for the LST appearance as these have sulfur atoms in their structure (**Figure 1**). Maujean (2001) described the thermal origin of volatile sulfur products in Champagne wines stored at 25°C in the dark could be formed by Strecker degradation of these sulfur amino acids [28]. Strecker's degradation of amino acids such as methionine and cysteine to aldehydes by α-dicarbonyl compounds formed during fermentation or oxidation contributes to the evolution of aroma in bottled wine [33]. Glyoxal (α-dicarbonyl compound generated during alcoholic and malolactic fermentation) reacts with methionine to form methanethiol and dimethyl disulfide, and also reacts with cysteine to form hydrogen sulfide, methanethiol, and other compounds [31, 32]. Methional, the initial product of Strecker's degradation of methionine, could be degraded via (retro-Michael mechanism) to form methanethiol, which is then oxidized to dimethyl disulfide [32]. Singlet oxygen, produced in photosensitized reactions, reacts with methionine resulting in the formation of dimethyl disulfide [34].

Maujean (1984) proposed that in white wine exposed to light, triplet riboflavin oxidizes methionine to methional, which then degrades to form methanethiol and dimethyl disulfide [35].

In relation to the amino acids degradation and the consequently formation of sulfur volatiles, the studies carried out in our laboratory (VITEC) confirm the data found in literature. **Figure 3** shows white wines bottled in clear glasses and exposed to three types of LED lights with different wavelength emissions. The concentrations of methionine, cysteine and the volatile sulfur compounds (as the sum of hydrogen sulfide, methantethiol, dimethyl sulfide and dimethyl disulfide), were determined after keep the wine in darkness, and after being exposed for 6 and 240 hours to different sources of light (L.1, L.2 and L.3). Results showed that the longer the light exposure time the lower the concentrations of the amino acids studied, and the higher the formation of volatile sulfur compounds. Furthermore, it should be noted that L.1 was the light that caused the greatest degradation of cysteine and methionine. As mentioned above, the nature of light is an important factor that can favor the LST default. In the next section, we can observe some examples.

#### *2.3.2 The volatile composition*

The LST is related to the formation of volatile sulfur compounds. Hydrogen sulfide, methanthiol, dimethyl sulfide, and dimethyl disulfide appear to be largely the main compounds responsible for the occurrence of this default [28]. All these compounds are mainly responsible for the formation of "reducing" aromas after bottling [30, 33, 36]. These compounds are characterized by unpleasant aromas in wines. On the one hand, within the thiol family is found hydrogen sulfide, which is a characteristic compound for providing wines with unpleasant aromas of rotten eggs, decomposing algae or wastewater. Other characteristic thiol of this defect is the mentioned methanthiol that contributes by descriptor aromas related to putrefaction smell and cooked cabbage. It should be noted that both compounds present a very low odor threshold (OT) values, corresponding to 1.6 μg/L and 0.3 μg/L respectively [37, 38]. On the other hand, within the family of sulfides and disulfides are found dimethyl sulfide and dimethyl disulfide, characteristic compounds for providing aromas associated with cabbage, asparagus, corn or onion flavors when present in high concentrations. They have an odor threshold around 25 μg/L and 29 μg/L, respectively [39–41]. The emergence of sulfur compounds related to the LST usually is also linked to a loss of fruity aromas of wines, such as ethyl and acetate esters, alcohols and fatty acids [42].

#### **Figure 3.**

*a) Sulfur amino acid content (cysteine and methionine) and b) volatile sulfur content, in white wines bottled in clear glass after being exposed to three types of LED lights (L.1, L.2, and L.3) during time (6 and 240 hours) compared to controls in the darkness.*

#### *The Light Struck Taste of Wines DOI: http://dx.doi.org/10.5772/intechopen.99279*

Moreover, the volatile sulfur compounds related to the LST of white wines is influence by the color of bottle. Here, some example comparing green bottles and clear bottles are shown (**Figure 4**). The types of source of light were also evaluated comparing six types of LEDs (**Figure 4**).

As can be seen in **Figure 4**, the white wine bottled in clear glasses presented higher concentrations of reduction aromas after 10 days of exposure with the LA, LC and LE lights, while the wine with the green bottle presented the highest concentrations with light LA. This is consistent with the degradation of riboflavin. The greater the degradation of riboflavin, the greater the presence of aromas of sulfur compounds in the wines (see Section 2.6). All this is due to the innovation of new LEDs which minimize or eliminate the emission of the region between 370 and 442 nm of the spectrum, thus reducing the risk of wine degradation (see Section 2.5 and 2.6).
